Diet and metabolism can affect how our bodies work in many ways, not just by turning excess calories into fat. Some metabolites act as signals or can be used to modify how genes are expressed, which can link diet to how our cells function and their capacity to respond to stress. We propose to study how one metabolite, s-adenosylmethionine (SAM), can both activate markers of immunity and limit how cells can change gene expression patterns in response to pathogens or other stress. These seemingly paradoxical functions occur because SAM can be used for different cellular needs. SAM can be used to make the phospholipid phosphatidylcholine (PC) and when PC is limited by the diet or needed for extra membrane production, much of the SAM is used for this biosynthetic process. However, SAM is also needed for modification of histones. Using C. elegans, we found that low SAM acted through low PC to activate markers of innate immunity on the standard laboratory diet. However, these same animals could not survive a bacterial challenge because they couldn't methylate histones priming gene activation and turn up pathogen responsive genes to sufficient levels. Thus, different contexts can change the phenotypes of low SAM, as cells need to prioritize utilization of this metabolite. Our proposal addresses several key questions. First, it is not known how the low SAM and PC signal activation of the immune system. Second, it is not understood how global chromatin modification under stress might change in low SAM and third, we don't yet understand how physiological regulators of low SAM might affect either of these phenotypes. Our C. elegans system is an excellent model for dissecting these mechanisms. We will combine genetic and molecular techniques (including whole genome assays for chromatin modification) with dietary modification to determine how SAM is linked to these phenotypes. We have used screens for SAM and PC-dependent modifiers of immunity to identify additional regulatory components and propose to determine how these candidates may be connected to immune activation. Furthermore, we have determined that multiple types of stress-induced gene expression depend on SAM and will use this system to ask how SAM and the histone methyltransferases utilizing it control transcription during stress. Although low SAM can cause phenotypes with very distinct molecular mechanisms, such as lipid-dependent activation of a MAP kinase in the immune response and modification of histones in transcriptional regulation, it is important to study these processes together. SAM depletion due to diet may impact either or both of these mechanisms, changing how our cells can respond to stress.